Sarcoma growth factor (SGF) is an EGF-like peptide released by murine sarcoma virus-transformed cells. It can reversibly induce the transformed phenotype in untransformed indicator cells. SGF is a potent mitogen that can interact with the EGF receptor system. Antibodies to the EGF receptor block the mitogenic effects of both EGF and SGF; similarly, cells lacking EGF receptors are unable to respond to either of these mitogens. SGF from serum-free conditioned media is purified to homogeneity using 5 column chromatography steps. The purified peptide is used to determine the relationship between SGF and EGF. The amino acid composition shows SGF is distinctly different from EGF. SGF has alanines, phenylalanines and lysines; whereas EGF is missing these residues. The EGF molecule has methionine and isoleucine residues which are missing in SGF. Amino acid sequencing of this peptide has identified residues as far as residue 42. If the first disulfide bonds of each molecule are aligned and a single space is introduced in the SGF molecule between residues 18 and 19 then 6 of the 34 known residues are identical for a 17.6% homology. If SGF is compared to human EGF (urogastrone) in a similar manner there are 10 identities for 29.4% homology.